Coronaviruses cause a variety of acute and chronic infections in both human and animals. Infection results in the apparent shutoff of host protein synthesis. The comparison of cell free extracts from mouse hepatitis virus (MHV)-infected and uninfected cells showed no difference in the translation of two model host genes, alpha-globin and CAT. However, translation of MHV mRNAs was significantly enhanced in the extracts from infected cells. The mechanism of this unusual translational enhancement was examined by constructing chimeric mRNAs containing the conserved 5' MHV leader sequence and the alpha-globin gene. These studies identified a cis-acting element within the 13 nt at the 3' end of the MHV leader sequence. In addition, the enhanced translation was only detected in extracts from infected cells, indicating that a viral protein functions in trans to enhance translation. The objectives of this proposal are to; l) identify the translational augmentor (TA) motif within the 3' end of the leader sequence. This will be accomplished by combination of linker scanning and both 5' and 3' deletions of the putative TA region; 2) identify the transacting viral factor by reconstitution of cell free extracts with viral proteins and transient expression assays; and 3) examine the mechanisms of both the augmentation of viral translation and the diminution of host cell translation by activation of p68 kinase and eIF-2 modification. These data will provide insights into the mechanisms regulating the replication of the unique class of animal viruses.